Push to Talk is the generic name for a range of services which enable users of mobile wireless handsets to communicate with one another almost instantaneously and at the push of a button, or at least at the push of a small number of buttons. An industry grouping is in the process of standardizing a Push to Talk service for introduction into present and future cellular networks including GSM with packet data services and 3G. The service is known as “Push to talk Over Cellular” (PoC).
PoC makes use of the IP Multimedia Subsystem (IMS) standardized by the 3rd Generation Partnership Project to facilitate the introduction of advanced data services into cellular networks, and in particular of real-time multimedia services. The IMS relies upon the Session Initiation Protocol (SIP) which has been defined by the Internet Engineering Task Force (IETF) for the setting up and control of multimedia IP-based sessions. FIG. 1 illustrates schematically the architecture of a cellular network which provides for PoC services between a number of user terminals or User Equipments (UEs) 1 in 3G parlance. UEs are attached to respective Radio Access Networks 2 which in turn are coupled to the IMS core 3. Within the IMS core 3, a number of servers are present including Serving Call Session Control Function (S-CSCF) servers 4 which are the main SIP servers that maintain session state for IMS services, and Proxy Call Session Control Function (P-CSCF) servers 5 which are the first points of contact for the UEs and which forward SIP messages to the S-CSCFs. The servers of the IMS core 3 are distributed within an operator's network and between networks. Additionally, a PoC server 6 is located within the IMS or is attached thereto. The PoC server may incorporate a Media Resource Function (MRF) node as defined by 3GPP.
FIG. 2 illustrates certain signaling associated with setting up a PoC session across the network of FIG. 1 (additional messages may also be transferred between the various nodes, although these are not shown in the Figure). A subscriber initiates a session by pressing the appropriate button on his/her terminal UE#1. This causes a SIP INVITE message to be sent to the peer terminal UE#2 via the PoC server in the IMS core, followed by the transfer of further signaling between the terminals and the IMS. As already mentioned, a key component of PoC is the near instantaneous connection of parties. Significant delays in transmitting speech are therefore to be avoided.
The time between the SIP INVITE message being sent and the IMS receiving an acceptance from the called party can be as much as 3 seconds due to fundamental properties of the network (e.g. paging, Temporary Block Flow (TBF) establishment, etc). In order to speed up the initial connection process, the initiating subscriber is therefore able to start talking upon receipt by his terminal of the SIP 202 Accepted message from the IMS (usually signaled to the initiating subscriber by the playing of a tone or “beep” on his terminal), even though the called party has not yet accepted the session. The initial talk burst may be buffered by a PoC server within the network until such time as it receives the SIP 200 OK message from the peer terminal. When that message is received, the talk burst is immediately sent to the peer terminal. Nonetheless, the delay perceived by the called party remains significant and it is desirable to reduce the delay still further.